Technical Field
The present invention relates to a semiconductor device, and in particular, relates to a technique that can be effectively applied to a semiconductor device having a thin film resistive element provided on an active element, with an insulating film therebetween.
Background Art
As a semiconductor device, switching power supply control ICs that control individual high breakdown voltage switching transistors, for example, are known. When such ICs are in an operational state, they form their own power supply by operating the high breakdown voltage switching transistor, but when the ICs are started up, it is necessary to provide a startup current from a startup circuit. Generally, startup circuits are integrated on the same semiconductor substrate as the switching power supply control IC, and as a result, the number of parts is reduced and the power supply system is simplified.
The startup current is attained by rectifying an inputted alternating current (AC) signal of 100 to 240V, and in order to feed this current to the startup circuit, it is necessary for the breakdown voltage of a normally-on element located upstream from the startup circuit to be 450V or greater. A lateral junction field effect transistor (JFET) with a high breakdown voltage is realized as a normally-on element in order for the normally-on element to be monolithically formed with the switching power supply control IC. The design specifications of the switching power supply device are decided according to the current driving performance of this element.
When the switching power supply device is unplugged from the outlet and stops receiving voltage supply from the AC input, then the primary side input voltage drops. If the switching power supply device continues to operate in this state, the ON time of the switching MOSFET becomes longer, which results in heat being generated therein. In order to prevent this problem, switching power supply devices are provided with a brownout function to stop the switching operation of the power supply when the input voltage is reduced.
The methods to realize this brownout function can be generally categorized into an external resistance dividing scheme and an IC chip embedding scheme. In the IC chip embedding scheme, high breakdown voltage dividing resistors are configured according to a voltage-withstanding structure of a high breakdown voltage device (startup element).
When embedding inside the IC chip, the voltage-withstanding structure of the already-existing startup element, which is the high breakdown voltage device, is used, and a spiral-shaped thin film resistor is built into a part of the structure. The thin film resistor is arranged in a spiral shape such that the potential gradually decreases along the periphery from the drain terminal disposed in the center of the startup element, which has the highest potential. The resistor is formed up to the source and gate regions, which are arranged so as to surround the drain region. Therefore, by being integrated with the startup element, it is possible to embed the resistor having the high breakdown voltage without providing a new voltage-withstanding structure (see Patent Document 1, for example).
When using a resistor voltage divider for input voltage detection, however, current is always consumed in the resistor voltage divider while input voltage is being applied. This is an issue that occurs regardless of whether or not the resistor voltage divider is embedded in the semiconductor integrated circuit, and it is normally possible to reduce power consumption by increasing the resistance of the entire resistor voltage divider.
However, when embedding this resistor voltage divider into an integrated circuit, the following two problems occur when increasing resistance.
The first is that, while it is possible to simply increase the number of times the thin film resistor is wound to increase the length thereof in order to increase the resistance, as the number of times the thin film resistor is wound increases, the plan view size thereof increases, and the area of the startup terminal is enlarged in order to accommodate the length of the thin film resistor. This enlargement of the startup element leads to an enlargement of the semiconductor chip, and therefore an enlargement of the semiconductor device. This results in a reduction in the number of chips that can be formed from one semiconductor wafer, which increases cost. The second problem is an increase in resistance variation as a result of dilution of impurity dosage in order to increase the resistance per unit length. One method to solve the second problem is to add an adjustable circuit such as a trimmer, but this results in a more complex circuit structure.
Thus, the inventors of the present invention focused on the leading out of the thin film resistor to arrive at the present invention. Accordingly, the present invention is directed to a scheme that substantially obviates one or more of the above-discussed and other problems due to limitations and disadvantages of the related art.